The earth's magnetic field has long been used to provide a reference from which to estimate headings and plan routes. Typically, a compass is used to measure the local magnetic field, and the earth's magnetic field is either assumed to be equal to the local field or is estimated from the local measurement using a combination of relatively manual factory calibration and user calibration processes. For example, a factory calibration method might include application of external fields in a controlled environment, and a user calibration method might include slowing rotating the compass through a planned series of maneuvers (e.g., providing another type of controlled environment).
These conventional calibration methods require time and/or expensive equipment to implement, and calibrations reached using these methods can rapidly become inaccurate or produce inaccurate headings during relatively uncontrolled motion and other short and long term changing environmental conditions, particularly when the earth's magnetic field includes a relatively large vertical component. GPS based systems have been developed that estimate headings from differential position data, but these types of systems are typically very inaccurate over short linear distances or quick changes in heading. Thus, there is a need for an improved methodology to address compass calibration that provides reliable heading information through a variety of changing environmental conditions.